Cattle feed can contribute to increased rural ozone levels

Cattle feed can contribute to increased rural ozone levels

As noted in prior posts, livestock operations are often associated with polluting streams and groundwater.  Manure lagoons not only are associated with odor, but because of the nitrogen and phosphorus produced are associated with dead zones in streams and estuarial habitats.  Additionally, prior posts have noted the perceived causes of the Gulf of Mexico dead zone, and its association with, among other things, runoff from farms and cattle operations.  Prior posts have also noted that ruminants can release copious amounts of methane, a greenhouse gas. 

In California, air quality regulators have been puzzling over the origin of rural ozone.  One thesis proposed has been that perhaps volatile emissions from animals and their wastes might be responsible for ozone formation when they mixed with nitrogen dioxide and other reactive organic air pollutants.  So, how might this happen?  Well, a ruminant's digestive system ferments grains, a byproduct of which is alcohol, a reactive organic gas that can drive the atmospheric chemistry responsible for creating ozone.  However, research has shown that the animals just did not create enough reactive organic gases.  See http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VH3-4S0YXTM-2&_user=10&_coverDate=07%2F31%2F2008&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1353474830&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=39172cda46f3ea8ae4cee82cb71c09e2.

The researchers then turned to the silage (feed) itself.  They sequentially tested each of seven different types of feed in a one-meter-square tented chamber.  Into this mobile cube they infused a mix of gases that matched the background composition of the air in California's San Joaquin Valley.  When they illuminated the chamber's interior with lamps to simulate sunlight's ultraviolet rays, the silage cooked up a lot of additional ozone even during a short six minute test.  Corn silage generated about 125 ppb ozone; alfalfa silage a little less; and, mixed oat-wheat silage, a significant 210 ppb.  Alcohols and aldehydes in silages have thus emerged as leading contributors to regional ozone formation. 

The ozone-formation potency of silage pales in comparison to that of tailpipe emissions from a gasoline-powered car.  However, the researchers calculated that the San Joaquin Valley has so much silage that animal feed would appear to be the single biggest driver of the region's smog-ozone problem.  Whereas cars and light-duty trucks can generate some 13 metric tons of ozone per day in the Valley, the researchers found that feed for the Valley's 10 million head of dairy cattle can produce another 24.5 tons of ozone per day (with corn silage accounting for all but about 8 percent of the feed's share).  The cattle themselves, through burps or emissions from waste, add only about 3 tons to the Valley's ozone per day. 

The geography of the San Joaquin Valley makes these finding somewhat unique since the Valley is bowl-like, not unlike the geography of Los Angeles.  This structure traps the emissions and allows for a higher generation of smog than would occur in a more flat geographic area with comparable animal loads. 

The report can be found at http://pubs.acs.org/stoken/presspac/presspac/full/10.1021/es902864u.